1. Field of the Invention
The present invention relates to an impact fastening tool such as an impact driver or an impact wrench.
2. Description of the Related Art
FIG. 10 schematically shows a block configuration of an impact driver as an example of an impact fastening tool. As can be seen from FIG. 10, the impact driver comprises a motor 1 as a driving source, and a strike mechanism 2 which generates an impact force by striking an anvil by a hammer and transmits a driving force of the motor 1 to an output shaft 3 with the impact force (not illustrated). Since the impact driver can perform a strong fastening work by its impact force and is splendid in workability because of high rotation and high torque, the impact driver is widely used in a building site or an assembly factory. Although it is not illustrated in particular, the strike mechanism 2 is comprised of a driving shaft rotatively driven by the motor 1 via a reducer (reduction gears), a hammer fitted to and rotated with the driving shaft, an anvil engaged with and rotated with the hammer, a cam mechanism which moves the hammer backward when a load equal to or larger than a predetermined reference value occurs in the anvil, and a spring for bringing the anvil to re-engage with the hammer with a strike when the anvil is disengaged from the hammer due to backward movement of the hammer. The output shaft 3 with a chuck 4 is integrally rotated with the anvil.
In FIG. 10, a numerical reference 5 designates a trigger switch. A rotation number of the motor 1, that is, a rotation number of the hammer and the output shaft 3 is controlled corresponding to a quantity of pulling the trigger switch. A numerical reference 6 designates a motor controller which uses a battery 7 as a power source and outputs a voltage set in the trigger switch 5 to the motor 1.
Japanese Laid-Open Patent Publication No. 2000-354976 proposes a method for controlling the fastening torque of such an impact driver that a fastening torque calculator for calculating a fastening torque T is provided, and the rotation of the motor 1 is stopped when the calculated torque T reaches to a predetermined reference value. The fastening torque calculator estimates the fastening torque T from a difference of kinetic energies before and after a strike of the hammer. This method is based on a relationship that an energy applied to the anvil provided at a root portion of the output shaft 3 by the strike of the hammer is substantially equal to an energy consumed in the fastening work.
Specifically, it is assumed that a relationship between a rotation angle θ of the anvil and the fastening torque T after a screw is completely fastened can be expressed in a function T=τ(θ) which is, for example, shown in FIG. 11, and it is further assumed that strikes by the hammer occur at points of rotation angles θ1, θ2, . . . θn. A value En which is an integration of the function τ in a section [θn, θn+1] designates an energy consumed in the fastening work, and is equal to an energy applied to the anvil by the strike of the hammer occurred at the point θn. Therefore, a mean value of the fastening torque T in the section can be obtained from the following equation (1) by using the integrated value En and a rotation angle Θn=(θn+1−θn) in an interval of the strikes of the hammer.T=En/θn  (1)
In order to control the fastening torque T, the driving of the motor 1 should be stopped at a time when a value of the fastening torque T becomes equal to or larger than a previously set torque Ts. The integrated value En can be obtained by the following equation (2) with using a mean rotation speed Ωn of the anvil in an interval of the strikes and a known moment of inertia Ja of the anvil.En=½×Ja×Ωn2  (2)
In addition, the mean rotational speed Ωn of the anvil in an interval of the strikes is obtained by dividing the rotation angle θn of the anvil in the interval of the strikes by an interval of the strikes of the hammer.
In case that the fastening torque T is obtained by using the above method including the equation (1), if a strike of the hammer, which does not really exist, is erroneously detected, the value of the calculated torque becomes inaccurate, so that the motor 1 cannot be stopped with the most suitable number of strikes of the hammer, consequently. Thus, since occurrence of the strike by the hammer must be detected precisely, a strike detector having high reliability is essential, thereby causing cost increase.
Therefore, Japanese Laid-Open Patent Publication No. 2001-246573 proposes a method for judging real or unreal of the occurrence of the strike of the hammer on the basis of the rotation speed of the output shaft 3 and rotation angle in an the interval of the strikes. However, when the impact fastening tool is actually used, various load fluctuations may occur. Thus, superficial phenomena such as the rotation of the output shaft 3 or the interval of the strikes may cause the reduction of reliability of the judgment result.